Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing. "Long" or "soft" x-rays (a. k. a. Extreme UV) (wavelength range of .lambda.=100 to 200 .ANG.) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection lens onto a wafer. Reticles for EUV projection lithography typically comprise a silicon or glass substrate coated with an EUV reflective multilayer material and an optical pattern fabricated from an EUV absorbing material that is formed on the reflective material. As is apparent, projection lithography systems are non-telecentric in that incident radiation from the condenser is not normal to the reflecting reticle surface.
In operation, EUV radiation from the condenser is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the EUV absorbing material. The reflected radiation effectively transcribes the pattern from the reticle to the wafer positioned downstream from the reticle. Conventional EUV lithographic projection system designs employ reflecting reticles that have flat surfaces to avoid contributing to distortions of the projected image. A scanning exposure device uses simultaneous motion of the reticle and wafer, with each substrate being mounted on a chuck that is attached to an X-Y stage platen, to continuously project a portion of the reticle onto the wafer through a projection optics. Scanning, as opposed to exposure of the entire reticle at once, allows for the projection of reticle patterns that exceed in size that of the image field of the projection system. Laser interferometry is typically used to determine the actual stage platen position.
Among the problems encountered in EUV projection lithography are point-to-point position variations caused by non-flatness in the reflective mask. One result is that the radiation reflected to the wafer is shifted relative to the ideal position. The art is in search of techniques to reduce such distortions.